The tests examined the content of the smoke for tar and carbon monoxide, as well as for the toxic chemicals nicotine, benzene and toluene.

“Cannabis smoke contains seven times more tar and carbon monoxide than tobacco smoke,” the institute’s magazine “60 million consumers” said.

Someone smoking a joint of cannabis resin rolled with tobacco will inhale twice the amount of benzene and three times as much toluene as if they were smoking a regular cigarette, the study said.

Smokers of pure cannabis leaves will also inhale more of these chemicals than from a normal cigarette, though the amount varies depending on the quantities.

“Smoking three joints every day — which is becoming frequent — makes you run the same risks of cancer or cardio-vascular diseases as smoking a packet of cigarettes,” the magazine said.

Cannabis is “by far” the most popular illicit drug in France, it said. The number of cigarette smokers and people drinking alcohol fell in 2005, while the number of cannabis users has increased in France over the past five years.